A majority of new cases of HIV are a result of sexual transmission between an HIV-negative and an HIV-positive person. The focus of prevention efforts has so far been mainly on changing the behavior of HIV-negative people who are at risk rather than that of HIV-positive people who can transmit to others. A better understanding of sexual risk behaviors of HIV-positive people may offer new opportunities for prevention. Research on people at risk for HIV infection consistently shows a relationship between substance use and sexual behavior, including high-risk sex. It is not known whether a similar relationship holds among people who are HIV-positive, and little is known about the underlying reasons for the substance use/sexual behavior relationship in people who are HIV-negative. In this two-year study, the investigators will examine sexual risk behavior, substance use, and the basic processes underlying these behaviors in a national sample of 1421 adults receiving health care for HIV. Subjects are participants in the HIV Cost and Services Utilization Study (HCSUS), a portion of whom were sampled and interviewed in the fall of 1998 as part of the Risk and Prevention study. Risk and Prevention collected detailed information about participants' sexual risk taking, including their own and their partner's use of alcohol and drugs before or during sex. Behaviors intended to reduce risk were also measured. Linked data from the HCSUS core study and a mental health supplement provide detailed data on substance use history and dependency, sensation seeking, depression, post-traumatic stress disorder (PTSD), and other person characteristics that may explain the substance use/sex relationship. Investigators will test the relationship between drug use, alcohol use, and various sexual behaviors; test the extent to which these relationships can be explained by the direct or indirect effects of person characteristics; and test whether sex-related motives, attitudes, and beliefs moderate the relationship between substance use and sexual risk.